This invention relates to the manufacture of synthetic linear condensation polyester feed yarns for texturing processes, and is more particularly concerned with a method of producing feed yarns which will provide more uniform dyeability in products prepared from them.
Processes for preparing polyester yarns from ethylene glycol and dimethyl terephthalate by ester-interchange followed by polymerization, or from ethylene glycol and terephthalic acid by direct esterification followed by polymerization, are well known. A major problem has been to control these processes to provide uniform dyeability in products made from the yarns. Textile mills must be able to put yarns from different yarn packages into adjacent areas of the same fabric and then dye the fabric without obtaining "dye junctions" (color discontinuities) which detract from the appearance of the dyed fabric. In other words, yarns from different packages must be "mergeable". Yarn manufacturers have developed a system of selling yarns of equivalent dyeability under a given "merge number". When a process change occurs which affects the dyeability, a different merge number is assigned. However, the textile mills have had a constant problem in scheduling the use of packages having different merge numbers and in preventing accidental mixing of packages.
Control of dyeability when producing polyester texturing feed yarns is a particularly difficult problem. When the process for producing such yarns includes a separate drawing step, the dyeability can be adjusted somewhat by making adjustments in the draw ratio used. Increasing amounts of partially oriented feed yarns are now being produced by high speed spinning, e.g., as disclosed in Piazza and Reese U.S. Pat. No. 3,772,872. These yarns are draw-textured and no practicable way has been found to control dyeability by making adjustments in the draw-texturing process.
Normally, in the production of polyester textile yarns, an objective has been to avoid the presence of aliphatic ether groups in the polymer because of their undesirable effect on properties of the yarns. Processes have been devised to make possible reduction of ether content to less than 3 mole percent, e.g., as illustrated by Izard U.S. Pat. No. 2,534,028, Mellichamp U.S. Pat. No. 3,496,146 and Armstrong et al. U.S. Pat. No. 3,534,082.
The unexpected discovery has now been made that uniform dyeability can be achieved by controlled addition of diethylene glycol at levels less than about 3 mole % without significant effect on other properties of textile yarns.